PROJECT SUMMARY Recovery after spinal cord injury (SCI) depends on plasticity of the nervous system, both spontaneous and induced by task-specific training. Traditional activity-based paradigms produce modest functional gains. However, pronounced deficits in eccentric motor control persist, making independent ambulation impossible for most individuals with SCI. Eccentric-focused downhill treadmill training is a novel, challenging intervention designed to promote greater skill learning of locomotion. Interestingly, MRI studies in healthy rodents and humans show that motor skill learning is accompanied by white matter plasticity along tracts activated by the training paradigm. Further, animal studies show that differentiation of oligodendrocyte progenitors, known as NG2 cells, and subsequent new myelin formation are required for learning of a motor skill. This proposal will employ a novel experimental design that uses similar outcome measures (MRI, 3D Kinematics) to examine white matter plasticity and motor learning in humans alongside mechanistic animal experiments following an equivalent downhill training paradigm. Because white matter plasticity and motor learning are understudied in both rodent and human SCI, the proposal aims to determine the role of newly formed myelin in recovery and motor learning after SCI in rodents (Aim 1) as well as the extent of myelin plasticity induced by motor learning in animals and humans with SCI (Aim 2). In Aim 1, we will use transgenic reporter mice that allow fate mapping of NG2 cells along with novel multicomponent T2 relaxation imaging (MCRI) to identify myelin formed in response to eccentric-focused downhill training. Additionally, we will use a strong proof-of- principle design in which we will examine training-induced motor learning after SCI in the absence of new myelin. Aim 2 will leverage an ongoing clinical trial examining downhill treadmill training after SCI in humans along with MCRI obtained from animals in Aim 1 to determine the extent of myelin plasticity induced by motor learning across species. Preliminary data in rodents show that downhill training mitigates the persistent eccentric motor control deficits after SCI and improves overground locomotion. Thus, eccentric-focused training may indeed be novel and challenging enough to drive white matter plasticity and motor learning. Therefore, we hypothesize that task-specific interventions that are novel and challenging, in this case downhill treadmill exercise, improve locomotor recovery across species via active myelination of spared CNS axons remote from the SCI.